Dual-cure resins and related methods

ABSTRACT

The present disclosure relates generally to curable resins, in particular dual-cure resins, and related methods for use in an additive fabrication (e.g., 3-dimensional printing) device.

BACKGROUND

Additive fabrication, e.g., 3-dimensional (3D) printing, providestechniques for fabricating objects, typically by causing portions of abuilding material to solidify at specific locations. Additivefabrication techniques may include stereolithography, selective or fuseddeposition modeling, direct composite manufacturing, laminated objectmanufacturing, selective phase area deposition, multi-phase jetsolidification, ballistic particle manufacturing, particle deposition,laser sintering or combinations thereof. Many additive fabricationtechniques build objects by forming successive layers, which aretypically cross-sections of the desired object. Typically each layer isformed such that it adheres to either a previously formed layer or asubstrate upon which the object is built. In one approach to additivefabrication, known as stereolithography, solid objects are created bysuccessively forming thin layers of a curable polymer resin, typicallyfirst onto a substrate and then one on top of another. Exposure toactinic radiation cures a thin layer of liquid resin, which causes it toharden and adhere to previously cured layers or to the bottom surface ofthe build platform.

Every additive manufacturing technology requires some form ofspecialized material. Additive manufacturing techniques using light tocure a liquid material, such as stereolithography (SLA and DLP), into anobject require photocurable materials.

Many current additive manufacturing materials are formed from polymeric(meth)acrylates. (Meth)acrylates are useful in 3D printing applicationsbecause the monomers and oligomers are highly reactive through radicalphotopolymerization. This reactivity allows for the printing process toproceed more quickly and efficiently with a higher degree of accuracy.However, the reactivity may also introduce less favorable qualities orlimit the types of qualities available in a 3D printing process. The endmaterial may be brittle because the resulting polymer is generallyinhomogenous and highly crosslinked. As additive manufacturing pushes tobe applicable in more functional prototyping or end-use applications,the material capabilities of (meth)acrylate based polymers become alimiting factor.

Accordingly there is a need in the art for new materials and expandedmaterial properties for applications of additive manufacturing.

SUMMARY

The present disclosure relates generally to curable resins, inparticular dual-cure resins, and related methods for use in an additivefabrication (e.g., 3-dimensional printing) device.

According to one or more embodiments, dual-cure resins for use inadditive manufacturing are provided.

In some embodiments, the dual-cure resin may comprise a photo-curablecomponent, configured to cure when subjected to an effective amount ofactinic radiation, and a secondary component. The secondary componentmay comprise a first secondary precursor species and a second secondaryprecursor species. The first secondary precursor species may beconfigured to be physically isolated or substantially physicallyisolated from the second secondary precursor species until subjected toan initiating event (e.g., a dissolving event or a degrading event) thatallows the first and second secondary precursor species to mix and cure.The initiation event may be a stimulus such as heat, mechanical force(e.g., sonication), addition of a catalyst, or some other mechanism.

In some embodiments, the secondary component may comprise a plurality ofparticles comprising a first secondary precursor species, wherein theplurality of particles are configured to dissolve when subjected to adissolving event.

In some embodiments, the secondary component may comprise a plurality ofencapsulants containing a first secondary precursor species, wherein theplurality of encapsulants are configured to degrade, when subjected to adegrading event, and release the first secondary precursor species forsecondary curing.

According to one or more embodiments, methods of producing anadditively-manufactured article are provided. In some embodiments, themethod may comprise providing a dual-cure resin comprising aphoto-curable component and a secondary component, the secondarycomponent comprising a plurality of particles comprising a firstsecondary precursor species, the secondary component further comprisinga second secondary precursor species. The method may further comprisesubjecting the photo-curable component to actinic radiation to produce aphoto-cured polymer. The method may further comprise subjecting thesecondary component to a dissolving event to dissolve the plurality ofparticles. The method may further comprise reacting the first secondaryprecursor species with the second secondary precursor species to producea secondary polymer, wherein the photo-cured polymer and the secondarypolymer form an additively-manufactured article.

In some embodiments, the method may comprise providing a dual-cure resincomprising a photo-curable component and a secondary component, thesecondary component comprising a plurality of encapsulants containing afirst secondary precursor species, the secondary component furthercomprising a second secondary precursor species.

The method may further comprise subjecting the photo-curable componentto actinic radiation to produce a photo-cured polymer.

The method may further comprise subjecting the secondary component to adegrading event to degrade the plurality of encapsulants and release thefirst heat-curable precursor species.

The method may further comprise reacting the first secondary precursorspecies with the second secondary precursor species to produce asecondary polymer, wherein the photo-cured polymer and the secondarypolymer form an additively-manufactured article.

The foregoing is a non-limiting summary of the invention, which isdefined by the attached claims.

BRIEF DESCRIPTION OF DRAWINGS

Various aspects and embodiments will be described with reference to thefollowing figures. It should be appreciated that the figures are notnecessarily drawn to scale. In the drawings, each identical or nearlyidentical component that is illustrated in various figures isrepresented by a like numeral. For purposes of clarity, not everycomponent may be labeled in every drawing.

FIGS. 1A-1B depict an illustrative additive fabrication system,according to some embodiments;

FIGS. 2A-2C depicts an illustrative additive fabrication system,according to some embodiments;

FIG. 3A depicts a schematic of a prevented chemical reaction due to thepresence of a blocking agent bonded to one of the reactants;

FIG. 3B depicts a schematic of a chemical reaction by which a blockingagent is removed;

FIG. 4 depicts a schematic of polyurethane formation, according to oneor more embodiments;

FIG. 5 depicts a schematic of polyurea formation, according to one ormore embodiments;

FIG. 6 depicts a representation of the chemical structure ofdicyandiamide, according to one or more embodiments;

FIG. 7 depicts a schematic of dicyandiamide in a latent stage, accordingto one or more embodiments;

FIG. 8 depicts a schematic of dicyandiamide during particle dissolution,according to one or more embodiments;

FIG. 9 depicts a chemical structure of a diisocyanate monomer species,according to one or more embodiments;

FIG. 10 depicts a chemical structure of a diisocyanate oligomer species,according to one or more embodiments; and

FIG. 11 depicts a schematic of an encapsulation process of a chemicalspecies in wax shells, according to one or more embodiments.

DETAILED DESCRIPTION

The present disclosure relates generally to dual-cure resins and relatedmethods for use in an additive fabrication (e.g., 3-dimensionalprinting) device.

As discussed above, in additive fabrication, a plurality of layers ofmaterial may be formed on a build platform. To illustrate one exemplaryadditive fabrication system, an inverse stereolithographic printer isdepicted in FIGS. 1A-B. Illustrative stereolithographic printer 100comprises a support base 101, a display and control panel 108, and areservoir and dispensing system 104 for storage and dispensing ofphotopolymer resin. The support base 101 may contain various mechanical,optical, electrical, and electronic components that may be operable tofabricate objects using the system.

During operation, photopolymer resin may be dispensed from thedispensing system 104 into container 102. Build platform 105 may bepositioned along a vertical axis 103 (oriented along the z-axisdirection as shown in FIGS. 1A-B) such that the bottom facing layer(lowest z-axis position) of an object being fabricated, or the bottomfacing layer of build platform 105 itself, is a desired distance alongthe z-axis from the bottom 111 of container 102. The desired distancemay be selected based on a desired thickness of a layer of solidmaterial to be produced on the build platform or onto a previouslyformed layer of the object being fabricated.

In the example of FIGS. 1A-B, the bottom 111 of container 102 may betransparent to actinic radiation that is generated by a radiation source(not shown) located within the support base 101, such that liquidphotopolymer resin located between the bottom 111 of container 102 andthe bottom facing portion of build platform 105 or an object beingfabricated thereon, may be exposed to the radiation. Upon exposure tosuch actinic radiation, the liquid photopolymer may undergo a chemicalreaction, sometimes referred to as “curing,” that substantiallysolidifies and attaches the exposed resin to the bottom facing portionof build platform 105 or to an object being fabricated thereon.

FIG. 1A-B represent a configuration of stereolithographic printer 101prior to formation of any layers of an object on build platform 105, andfor clarity also omits any liquid photopolymer resin from being shownwithin the depicted container 102.

Following the photo-curing of a layer of material, a separation processis typically conducted so as to break any bonds (e.g., adhesive bonds)that may have been produced between the cured material and the bottom111 of container 102. As one example, build platform 105 may be movedalong the vertical axis of motion 103 in order to reposition the buildplatform 105 for the formation of a new layer and/or to imposeseparation forces upon any bond with the bottom 111 of container 102. Inaddition, container 102 is mounted onto the support base such that thestereolithographic printer 101 may move the container along horizontalaxis of motion 110, the motion thereby advantageously introducingadditional separation forces in at least some cases. An optional wiper106 is additionally provided, capable of motion along the horizontalaxis of motion 110 and which may be removably or otherwise mounted ontothe support base at 109.

According to one or more embodiments, FIGS. 2A, 2B depict a schematic ofan inverse stereolithographic printer 200. In the example of FIGS. 2A-B,stereolithographic printer 200 comprises a build platform 205, acontainer 202, a leveling mechanism 201, and liquid resin 212. Thedownward facing build platform 205 opposes the floor of container 211,which contains photopolymer resin 212.

FIG. 2A represents a configuration of stereolithographic printer 200prior to formation of any layers of a part on build platform 205. Someor all of the photopolymer resin located between the build platform andthe bottom of the container may be cured (e.g., by directing actinicradiation through the base of the container onto the resin as describedabove).

As described above, stereolithographic printers 100 and 200 shown inFIGS. 1A-B and FIGS. 2A-B, respectively, may cure layers of photopolymerresin in contact with both a desired build surface (e.g., the buildplatform 105 or 205 and/or a previously formed layer of material) and anopposing surface 111 or 211. Such an approach may be employed in systemssometimes known as “inverted” stereolithography machines, where actinicradiation is introduced through an optical window in the bottom of acontainer.

To illustrate one exemplary additive fabrication technique in which apart is formed in contact with a surface other than another layer or thebuild platform, an inverse stereolithographic printer is depicted afterforming several layers of an object 203 in FIG. 2B.

According to one or more embodiments, the liquid resin may comprise acombination of oligomers, monomers, and photoinitiators. This basicliquid resin could optionally include pigments, dyes, and otherspecialty additives as well. The liquid resin may comprise a firstcomponent comprising one or more of the types of species listed above,as well as others. The first component of the liquid resin may beconfigured to cure upon exposure to actinic radiation. The liquid resinmay further comprise a second component comprising one or more of thetype of species listed above or others. The second component of theliquid resin may be configured to cure in response to an initiatingevent (e.g., application of heat). Liquid resins that include both firstand second curing components are referred to herein as dual-cure resins.In one embodiment the liquid resin is dispensed from the dispensingsystem 104 into the container 102. With everything in place as discussedabove with printer 100 or 200, the source of actinic radiation can beused to expose the liquid resin. The actinic radiation would cause thephotoinitiators to form photo-reactive species. These photo-reactivespecies may then react with monomeric or polymeric components andinitiate a cross-linking or further polymerization reaction. Thispolymerization causes the layer to cure in the cross section exposed toactinic radiation. This cross section adheres to either the buildplatform 205 or to the previous layer of the object 203. The objectformed would be made of a polymeric material. The type and properties ofthe material would depend on the monomeric, oligomeric, and/or polymeric(meth)acrylates used as the base of the resin.

FIG. 2C shows an object 220 in a green state, according to one or moreembodiments. The object 220 may be formed by the process of formingsuccessive photo-cured layers 203, as discussed above with regard toFIG. 2B. The object 220 in the green state may comprise the photo-curedpolymeric material 225, as well as a first precursor species 230 and asecond precursor species 235 of a secondary component that has not yetundergone substantial secondary polymerization, i.e., is in a latentstate. In the green state, object 220 has some defined structure givenby the photo-cured polymer 225, but has not yet been fully cured toprovide an object in its final state. The secondary component may besubjected to an initiating event 240 (e.g., application of heat) whichcauses a secondary polymerization process to occur to produce article250. The final article comprises both the photo-cured polymer species orpolymeric network 225, as well as the secondary polymer species orpolymeric network 255. Mechanisms related to secondary curing arediscussed further, herein.

Dual-cure resins for use in additive manufacturing are generallydescribed herein. As used herein, the term “dual-cure resin” refers to aresin having at least two separate curing steps that may occursimultaneously or sequentially. For example, according to someembodiments of the invention, a dual-cure resin may comprise a firstcomponent configured to cure when subjected to actinic radiation and asecond component in which curing is initialized through a separateevent, such as the application of heat. Other mechanisms forinitializing curing are also possible, as discussed herein.

Incorporating a secondary polymerization mechanism may make many morepolymeric chemistries accessible to additive manufacturing leading tooutstanding and diverse mechanical properties that would be difficult orimpossible to achieve in a single-cure resin. According to one or moreembodiments, the primary or initial cure step would rely on traditionaladditive manufacturing techniques to create a scaffolding or greenarticle. Once the initial reaction constructs the desired object, thesecondary reaction may take place and the desired properties, such asincreased strength, may be attained. In some embodiments, a two-stepcure may allow for time to adjust the shape or placement of the materialwhile it is in a gel-like form before subjecting the article to thesecondary cure mechanism that would more completely harden the material.

Embodiments presently disclosed fulfill a need in the industry for newpolymeric materials with longer-term shelf stability and enhanced rangesof material properties. Some embodiments disclosed herein accomplishthese goals by physically isolating a species of the secondary curecomponent such that it is entirely or substantially non-reactive untilan initiating event (e.g., application of sufficient heat). Keeping theprecursor species of the secondary cure physically isolated from oneanother at ambient conditions allows for the initial cure step to form a“green” structure or scaffold in the desired shape that can then undergothe secondary cure process to finalize the mechanical properties of thematerial, according to some embodiments.

The dual-cure resin may comprise a primary component for forming a firstpolymer and a secondary component for forming a second polymer. Each ofthe components may comprise one or more precursor species that cure, orreact to form polymers (e.g., (meth)acrylate or polyurea), upon theinitiation of certain conditions.

The primary component may comprise a photo-curable component configuredto cure when subjected to an effective amount of actinic radiation. Thecured polymeric material may be a type of (meth)acrylate or acrylatepolymer. (meth)acrylates are useful in 3D printing applications becausethe monomers and oligomers are generally stable in ambient conditions.The photo-curable component may further comprise photo-initiator. Withthe addition of a photoinitiator and the relevant light source, thematerials may become highly reactive through radicalphotopolymerization, and cure to form a polymer. The article formed atthis stage may be referred to as a “green” article, indicative of thefact that further curing and strengthening may take place.

The resin may further comprise a secondary component configured to cure,or react to form a secondary polymer, upon the initiation of certainconditions. The secondary polymer may comprise a first secondaryprecursor species and a second secondary precursor species. According tocertain embodiment, the first secondary precursor species is configuredto be physically isolated or substantially physically isolated from thesecond secondary precursor species until subjected to an initiatingevent that allows the first and second secondary precursor species tomix and cure. The initiation event may be a stimulus such as heat,mechanical force (e.g., sonication), addition of a catalyst, or someother mechanism.

In some embodiments, isolating a precursor species of the secondary curecomponent allows the initial or primary cure reaction (e.g.,photoreaction) to proceed without interference, while maintaining thesecondary cure precursor species dispersed in the initially curedobject. Once the initiation factor is provided, the active compound ofthe secondary cure is released and the secondary cure reaction canproceed. In some embodiments, isolation of a secondary precursor species(either through encapsulation or particularization) allows the resin tobe created in a single cartridge system reducing waste and improving thecustomer experience.

Dual-cure resins disclosed herein may have long shelf and/or pot life.Delay of secondary curing (via physical isolation of a precursorspecies) may allow for a longer pot and/or shelf life for the dual-cureresin. As a result of its long shelf life, according to someembodiments, dual-cure resins disclosed herein may be manufactured andsold with the first and second cure components both already disposed ina single resin composition (e.g., in a single cartridge), therebyreducing the impact of human error or other complications related toproperly mixing two compositions. In embodiments in which the first andsecond components are mixed post-sale by a consumer, the resultingliquid resin may have a shelf life and/or pot life of at least a week, amonth, six months, or a year.

Prior to curing, the first and second secondary precursor species mayreside in the same resin (for example, in the same pot or in the samechamber of a cartridge, or in the same “green” article), while the firstsecondary precursor species remains physically isolated or substantiallyphysically isolated from the second secondary precursor species, therebypreventing premature curing. For example, the first precursor species ofthe secondary component may be encapsulated by a shell which presents aphysical barrier physically isolating it from the second precursorspecies to prevent curing. In another example, the first species ispresent in the resin in an insoluble particle or a powder form, allowingonly a small fraction of that precursor species (e.g., the smallpercentage of material at the surface of each particle) to be exposed toanother precursor species, thereby substantially physically isolatingthe precursor species from one another. The physical isolation of theprecursor species may then be eliminated upon the occurrence of aninitiating event, allowing the precursors to mix and cure. For example,where one of the precursor species is encapsulated, the initiating eventmay be a degrading event—an event which causes the encapsulating shellto degrade and release the formerly physically isolated precursorspecies. In embodiments in which one of the precursors is suspended inparticle form, the initiating event may be a dissolving event—an eventwhich causes the particle comprising the precursor species to dissolveand release the formerly substantially physically isolated precursorspecies. The degrading or dissolving event may comprise any ofapplication of heat or mechanical force (e.g., vibrational force), orintroduction of a chemical species (e.g., a solvent or catalyst). Othermechanisms are also possible.

Embodiments described herein, in which different precursor species of asecondary component are physically isolated from one another even whereboth are present in the same resin mixture to prevent curing, may bedistinguished from alternative mechanisms in which precursor species areprevented from curing due to a blocking chemistry mechanism. Resincompositions employing blocking chemistry bond a blocking functionalgroup to one of the precursor species to prevent it from reacting with asecond precursor species and curing. Once it is time to allow curing totake place, an intermediary reaction is triggered, for example, by theapplication of heat, to react and remove the blocking agent, allowingthe two precursor species to proceed to cure. The presence of a blockergroup on a precursor species does not constitute it being physicallyisolated, at least as that term is used herein.

Using blocked polymers for additive manufacturing application risksleaving small molecules in the final object, at least in someembodiments. These small molecules can be plasticizing agents.Plasticizer may affect the material properties or leach out of thefinished object over time. Such leaching may make the object unsuitablefor certain applications such as medical or food grade materials.Furthermore, potential solutions for removing blocking molecules mayresult in additional problems, such as reducing the speed at which thesecondary cure step can proceed.

The approach of blocking chemistry has three primary disadvantages. (1)If the small molecule does not remain bound to the backbone afterdeblocking, it may act as a plasticizer potentially leaching out of thecured object affecting mechanical properties, biocompatibility, and mayimpact available applications overall. (2) Where the small molecule isbound in the backbone of the polymer network, the resulting reaction mayhave a slow cure rate and/or require a high curing temperature, as theblocking reaction will continue in the forward and reverse directionrequiring long curing times and substantially high temperatures (3) Theblocked isocyanate could be a urethane or urea compound, which is, byits nature, much higher in viscosity than its unblocked isocyanatecounterpart due to hydrogen bonding with itself. For this reason, mostcommercially available blocked isocyanates (such as Baxenden Trixene BIseries) have a very high viscosity for relatively low isocyanatecontents. As such, only small amounts of the blocked isocyanates can bereasonably included in the resin while keeping the viscosity atprintable levels. This limits the concentration of urethane/urea bondsin the postcured polymer network and thus limits the resulting materialsproperties. Additionally, with conventionally available blockedpolymers, the reaction proceeds in ambient conditions at some level.This increases the viscosity of the material and decreases the pot life.

FIG. 3A shows a schematic of chemical reaction prevented from takingplace because of the presence of a blocking agent bonded to one of thereactants. In this case, some blocking agent “B” is attached to theisocyanate “X” and prevents it from reacting with the alcohol “Y” (oramine) to make a urethane (or urea) at ambient temperature. The sameblocking principle could, of course, be applied to reactants differentfrom those of this example.

FIG. 3B shows a schematic of a chemical reaction by which the blockingagent of FIG. 3A is removed from the isocyanate “X” by the applicationof heat, which would in turn allow the reaction to take place, which hadpreviously been prevented by the blocking agent B as shown in FIG. 3A.The presence of the cleaved blocking agent B, however, can have anundesired plasticizing effect.

As already mentioned, the physical isolation or separation of secondaryspecies may be accomplished with the use of encapsulants. According toone or more embodiments, the secondary component comprises a pluralityof encapsulants containing a first secondary precursor species, whereinthe plurality of encapsulants are configured to degrade, when subjectedto a degrading event (e.g., application of an effective amount of heat),and release the first secondary precursor species for secondary curing.The encapsulant may comprise, for example, a wax or polymer shell. Theencapsulant could be, for example, a wax or polymer shell that ruptureswith phase transition or expansion of the inner encapsulated materialwhen sufficient heat is applied. The secondary component may furthercomprise a second secondary precursor species configured to react withthe released secondary precursor species to produce a secondary polymer.

The substantial physical isolation or separation of secondary speciesmay, alternatively or additionally, be accomplished with the use ofparticles that are insoluble in the resin at ambient conditions. Forexample, according to one or more embodiments, the secondary componentcomprises a plurality of particles, in turn, comprising a firstsecondary precursor species. The plurality of particles may beconfigured to dissolve when subjected to a dissolving event (e.g.,application of an effective amount of heat). Until subjected to thedissolving event, however, the plurality of particles may be configuredto remain substantially undissolved. The insoluble particles may remainsuspended and dispersed in the liquid photopolymer resin as it ishardened into the green article during the initial cure step. Once theinitiation factor is provided (e.g., sufficient heat is applied), theparticles may become more soluble in the hardened material, allowing thedissolved precursor species to mix with other precursor species andallow the second cure reaction to begin.

According to one or more embodiments, the different precursor speciesmay be selected to form a desired secondary polymer. The secondarypolymer may be a thermoset plastic. The secondary polymer may be, forexample, a polyurea, a polyurethane, and/or an epoxy. Other secondarypolymers are also possible.

The use of secondary polymers in addition to the primary polymer (e.g.,methacrylate) can facilitate an expansion of available materialproperties. For example polyureas tend to have more desirable chemical,heat response, and resistance to aging. Additionally polyureas can bestronger with a higher elongation making the resulting material tougherand better able to withstand repeated use. Additionally, polyepoxides(epoxies) are a class of thermosetting polymer with desirable mechanicalproperties such as, temperature and chemical resistance. The followingdiscussion relates to polyureas, polyuretheanes, and polyexpoxides, butcan be applied to any number of polymer groups as one skilled in the artwould understand.

In some embodiments, the secondary cure component may be configured toform a polyurea upon curing.

In conventional polyurea synthesis, a two-step approach is typicallyused. For example, first, a small molecule diisocyanate monomer isreacted in excess with a large difunctional amine. Second, anamine-based chain extender is added to form the complete polymer. Anexample of a conventional polyurea synthesis is shown in FIG. 5. In someembodiments an optional amine accelerator may also be incorporated.

In embodiments of the present invention, where the secondary polymer ispolyurea, at least one of the precursor species may be fully orsubstantially physically isolated from a second precursor species. Forexample, the first precursor species may be present in the form ofinsoluble particles. Alternatively, the first precursor species may beencapsulated.

The first precursor species (with limited exposure) may comprise anamine. It may comprise a polyamine. It may comprise a diamine, it maycomprise dicyandiamide (DICY). The first precursor species (e.g., DICY)may be present in particle form or encapsulated. In some embodiments,the first precursor species may comprise 3,3′ diaminodiphenyl sulfonehigh temperature aromatic amine curing agent.

The second precursor species (which is generally, although notnecessarily, not isolated within the resin) may comprise an isocyanatespecies. For example, the second precursor species may comprise apolyisocyanate species. The second precursor species may comprise adiisocyanate species. Upon being introduced to each other the first andsecond precursor species react to form polyurea.

In some embodiments, the amine reactant may comprise an amine reactantthat is present as an insoluble particle at ambient temperature andstarts to dissolve at elevated temperatures. Increasing the temperaturemay improve the solubility and diffusion of the amine reactant whichallows it to react. In some embodiments, dicyandiamide may be the aminereactant.

Use of DICY, instead of alternative traditional polyols or polyamines toform a a secondary polymer, may have several advantages. First,DICY-isocyanate based resins may be highly stable at ambient temperatureand confer a much longer potlife (e.g., a pot life of several months oryears). Second, use of DICY (or other particles) may be advantageousover use of liquid polyols which can lead to excessive leaching duringpostcure heating and even in the green state. Because DICY is a solidwith a high melting point, leaching is not a concern. Use of a smallmolecule diamine, such as DICY, may aid in achieving high localizedconcentrations of urea groups upon the reaction of the amines withisocyanates, which may lead to even higher toughness polymers afterpostcuring. (Small molecule polyols usually have low melting points andoften result in leaching at high temperatures.)

FIG. 6 depicts a representation of the chemical structure ofdicyandiamide, according to one or more embodiments. As shown in FIG. 6,dicyandiamide includes two amine groups.

FIG. 7 depicts a schematic of a secondary component 700 during a latentstage, according to one or more embodiments. A particle 710 contains aprecursor species 720. In this particular embodiment, the precursorspecies is dicyandiamide. The dicyandiamide 720 is present in particleform 710 and is substantially isolated from the companion precursorspecies 730, which in this embodiment comprises diisocyanate,substantially preventing the formation of a secondary polymer (e.g.,polyurea) to occur at levels that would interfere with the printabilityof the resin. Such an arrangement, allows for an extended pot-life orshelf-life of a dual cure resin containing the secondary component 700.

FIG. 8 depicts a schematic of a secondary component 700 after beingsubjected to a dissolving event (e.g., application of heat), accordingto one or more embodiments. The dissolution of particle 710 causes theprecursor species 720, which in this embodiment comprises dicyandiamide,to be released and to mix and react with the compoanion precursorspecies 730 (e.g., diisocyanate) to form a secondary polymer 740 (e.g.,polyurea). While the embodiments shown in FIGS. 7 and 8 incorporatedicyandiamide and diisocyanate, the same principal could be appliedusing different precursor species and/or to form a different secondarypolymer. Likewise, in some embodiments an initiating event other thanthe application of an effective amount of heat, could be used to causeparticle dissolution, such as addition of a catalyst.

In some embodiments, the secondary cure component may be configured toform a polyurethane.

In conventional polyurethane synthesis, a two-step approach is typicallyused. For example, first, a small molecule diisocyanate monomer isreacted in excess with a large difunctional polyol. This first step mayproduce oligomeric isocyanates. Second, a small hydroxyl-based chainextender is added to form the complete secondary polymer. In someembodiments, the chain extender may be a polyol species or a polyaminespecies. An example of a conventional polyurea synthesis is shown inFIG. 4. In some embodiments an optional accelerator may be incorporated.

In some embodiments configured to form polyurethane as the secondarypolymer, the first precursor species (e.g., the isolated species) maycomprise at least one of a chain extender and cross-linking agent. Insome embodiments the chain extender or cross-linking agent may beencapsulated. Chain extenders and crosslinkers are discussed furtherherein.

In embodiments configured to form polyurethane, a second precursorspecies (which may be not encapsulated, not in particle form, or nototherwise isolated from the rest of the resin) may comprise anoligomeric isocyanate species. Upon being introduced, the precursorspecies may react to form polyurethane.

In some embodiments, the secondary cure component may be configured toform a polyepoxide, or epoxy. The first precursor species (with limitedexposure) may comprise an amine. For example, it may comprise apolyamine. In some embodiments, the first precursor species may comprisea diamine. In some embodiments, the first precursor species may comprisedicyandiamide (DICY). The dicyandiamide may be present in particle formas discussed above, in related to dicyandiamide's potential use informing polyurea.

The second precursor species (generally, but not necessarily, present inan unenecapsulated or non-particle form) may comprise an epoxide.Potential epoxy precursor species include Epon 828 (bisphenol Adiglycidyl ether). In some embodiments, an amine accelerator may beincorporated into the process when forming epoxies, e.g. Ancamine 2442.Upon being introduced to each other after the initiating event, thefirst and second species may react to form an epoxy.

In embodiments where the secondary component comprises a plurality ofparticles, the particles may comprise dicyandiamide (DICY, also known ascyanoguanidine). DICY is a latent-curing agent which can be prepared asa powder dispersion in the liquid resin. For example, DICY may be usedas a precursor for polyurea or epoxy formation.

The chemical structure of DICY, and the fact that it is an insolublemicronized powder, may result in an extremely low reaction rate atambient temperature, both for epoxy-amine and isocyanate-amine curingmechanisms, allowing for improved shelf-life and pot-life. Someembodiments may incorporate micronized powders of DICY that arecommercially available, such as those available from AIR PRODUCTS underthe AMICURE brand name and from CVC THERMOSET SPECIALTIES under theOMICURE brand name. In some embodiments, the DICY powder may be OMICUREDDA5 from CVC SPECIALTIES, which has an average particle diameter ofabout 4 μm. The DICY powder may be used in combination with amineaccelerator agents, such as ANCAMINE 2442 from AIR PRODUCTS, which serveto lower the cure temperature of the curing reaction to 120-150° C.These accelerators are often solids ground to ultra-fine particle sizewhich are insoluble in resins at room temperature.

According to one or more embodiments, physical isolation of one or moresecondary precursor species may be achieved by encapsulation of thespecies. The precursor species may remain isolated in the encapsulant,until an event triggers the degradation of the capsule and release ofthe precursor species. In some embodiments the encapsulated precursorspecies may be a chain extender and/or crosslinker species. In someembodiments, upon release from the encapsulant, the chain extenderand/or crosslinker species may react with a precursor oligomer (e.g., adiisocyanate oligomer) to form polyurethane.

In some embodiments, precursor oligomers may be strengthened into apolymeric network by bonding with chain extender and/or crosslinkerspecies. The links between precursor molecules are formed by reactionwith chain extenders and/or crosslinkers that can react with functionalgroups from two or more separate oligomers. Both chain extenders andcrosslinkers are low-molecular multifunctional species: Difunctionalproducts can react to form a linear extended structure, and aregenerally referred to as chain extenders; Tri- and other multifunctionalentities can react to form a tridimensional lattice, they arecrosslinkers. However, in practice, sometimes usage of the two termsoverlaps.

During the formation of, for example, polyurethane, chain extendersand/or crosslinkers may be used to join the end groups of isocyanateoligomers. Chain extenders and/or crosslinkers may be used to limit, orextend the rotation and conformation of oligomeric molecules. They canalso be used to control the cross link density, or partialcrystallization of the final structure of the polymer formed. Both ofthese parameters control the ability for the oligomers to organize andphase separate into microdomain regions, which, in turn, controls thefinal mechanical properties of the bulk polyurethane. Chain extendersare generally highly reactive molecules, often containing either anamine in the structure itself or as a separate catalyst.

In some embodiments, short diamines, diols, and/or triols are used aschain extenders and comprise the core of capsules protected by lowmolecular weight polyolefins or waxes as the shell material. In someembodiments, the chain extenders may comprise triethylene glycol,glycerol, hexamethylenediamine, and/or triethanolamine. Some embodimentsmay further comprise an additional catalyst, for example, dibutyltindilaurate and/or DABCO.

In some embodiments, the encapsulant may comprise a polymer species. Insome embodiments, the encapsulant may comprise a polyolefin wax. In someembodiments, the encapsulant may be selected to have a particularmelting point. In some embodiments, the encapsulant is configured todegrade and release its contents when the composition is heated to reachthe melting point temperature, allowing for secondary curing and theformation of the secondary polymer (e.g., polyurethane network). Themelting point of the encapsulant should be sufficient to survive a rangeof shipping and storage temperatures, but not so high that damage to afirst stage cure (e.g., photocured methacrylate component) would beincurred prior to the encapsulant melting. The encapsulant size selectedmay depend on various parameters. In some embodiments, the encapsulantdiameter is from about 1 micron to about 50 microns.

Encapsulation of a secondary precursor species (e.g., chain extender)may be achieved through different processes, as would be understood by aperson of ordinary skill in the art. FIG. 11 shows a representativeencapsulation process 1100. In the process 1100, According to someembodiments, a step 1115 is to form an emulsion 1125 (e.g., water in oilemulsion) of the precursor species (e.g., chain extender) 1105 in a bulkliquid wax phase 1110 above its respective melting point. In asubsequent step 1130, this first emulsion 1125 may then be emulsified inan external aqueous (e.g., polyvinyl alcohol or “PVA”) solution 1120 toform a second emulsion 1135 (e.g., water in oil in water emulsion.

Emulsification may be performed with, for example, a T25 ultraturraxhomogenizer, a tip ultrasonicator, a membrane emulsifier, or amicrofluidic device. Cooling while maintaining the emulsion allows forsolidification of the wax shell and therefore, formation of capsules.The capsules may be washed with pure water to remove the surfactant andany surface contaminates. The capsules may be dried under ambientconditions or using any drying methods known in the art including butnot limited to low pressure partial vacuum drying, forced convectiondrying, spray drying, or lyophilization. The microcapsules may thenmixed into a composition with another secondary precursor species (e.g.,isocyanate oligomer) and a first, photocurable component (e.g., anacrylate-based component) at the proper stoichiometric amount or someamount more than the stoichiometric amount to ensure full reaction. Atthis stage, the composition may be ready for use in stereolithographicprinting.

In some embodiments, the wax shell may be degraded by heating thecomposition, after a first photocuring stage, allowing the chainextenders to mix and interact with the isocyanates bound into theacrylic network formed during printing and building a crosslinkedpolyurethane network inside the acrylic matrix.

The secondary component may comprise additional precursor species. Forexample, according to some embodiments, a second precursor species maycomprise an isocyanate species. In some embodiments, the isocyanatespecies may be an isocyanate-terminated oligomer. In some embodiments,use of an isocyanate oligomer may provide advantages such as reducedtoxicity as compared to small molecule isocyanates. Additionally, themulti-functional oligomers may allow for a greater breadth ofcharacteristics as the oligomeric components may be tailored forspecific properties. For example, in some embodiments, the oligomericcomponents may be tailored to allow for photopolymerization while theisocyanate-terminated region remains available for use in a secondreaction resulting in formation of a secondary polymer.

FIG. 9 depicts the chemical structure of an diisocyanate monomer [4,4′methylene diphenyl diisocyanate], according to one or more embodiments.FIG. 10 depicts the chemical structure of MDI-PolyTHF-MDI diisocyanateoligomer, according to one or more embodiments. The oligomers mayfurther comprise diol species. In some embodiments, the isocyanates(both 2,4 and 4,4 methylene diphenyl diisocyanate[MDI]) are reacted atsuitable stoichiometric ratios with different diols to controlmechanical properties. Longer more flexible diol molecules result inhigher elongation, often softer elastomeric urethanes. In someembodiments, the diols may be linear hydrocarbon diols, polyethyleneglycols [PEG] and polytetrahydrofuran [PTHF]. In some embodiments, thefirst NCO group on the MDI is far more reactive than the second. In someembodiments, limiting the amount of hydroxyl [OH] groups to a ratio of 2NCO per reactive OH or NH on the polyol or diamine allows for thecreation of oligomeric versions of the polyol chain capped with two MDIgroups each with one unreacted NCO moieties on either end.

In some embodiments, oligomeric formation may be done in bulk phase withagitation as well as heating and cooling, as needed, giving sufficienttime for the reaction to proceed to completion. The ratio of short rigidMDI molecule to flexible polyol may affect the properties of the finalpolyurethane. According to some embodiments, shorter linear polyolchains result in lower elongation, while longer linear polyol chainsresult in higher elongation more flexible materials.

Resin formulations may further comprise various additives. For example,in some embodiments, the resin may comprise treated and/or untreatedfumed silica (e.g., AEROSIL 200) to mitigate settling of particles orcapsules. Rubber toughening agents (e.g. Albidur EP 2240 A epoxy-coatedsilicone core-shell particles) may also be used in some embodiments.Other additives may also be used.

Pot life is the term of usability for a mixture typically determinedunder ambient conditions. Additive manufacturing techniques may requirea long pot life such that a mixture is usable for the duration of theprint. Print times may range from very short times of just a few minutesto multiple days. Additionally, it may be advantageous to extend the potlife much beyond the duration of a single print, such that the consumermay experience the greatest ease of use with minimal wasted material.

Shelf life is the term of usability for a mixture typically determinedfrom manufacture until final use, and under a variety of conditions andtemperatures that could be expected during shipping and storage, as wellas ambient conditions or in the printer itself. It may be advantageousto extend the shelf life of a resin to a sufficient length of time, soas to allow the resin to be pre-mixed at a manufacturing stage, ratherthan mixed by a user post-purchase. It may be advantageous to have ashelf life of one month, three months, a year, or longer.

Improved pot and shelf life of the resin is facilitated, in part, byhaving the secondary polymerization component remain dormant for longperiods of time under ambient conditions (e.g., ambient temperature andmoisture). Premature gelation or viscosity increases in the liquidresin, as the result, for example, of reaction in the presence ofmoisture, may reduce pot/shelf life. As portions of the secondarycomponent react, the viscosity increases, in turn impacting variousmechanisms of the additive manufacturing process such as separation ofthe object from the container and material mobility for printingsubsequent layers, and reducing the term of usability of the resin.

Accordingly, the methods and compositions described herein, in which afirst precursor species is fully or substantially physically isolatedfrom a second precursor species until an initiating event, may providefor improved pot and shelf life.

According to one or more embodiments, the liquid resin may have aviscosity of from about 1 cP to about 10,000 cP, or of from about about1,000 cP to about 5,000 cP, when measured at a temperature 30 □. Othervalues are also possible.

In some embodiments, the photocurable component and the secondarycomponent are present in the dual-resin formulation at certain ratios byweight. For example, in some embodiments, the ratio by weight ofphotocurable component to secondary component is from about 20:80 toabout 80:20, or from about 40:60 to about 60:40 or at about 50:50. Insome embodiment the ratios of first component to second component may beselected to balance the desire for high secondary polymer content to thedesire for a high strength in the “green” state (i.e. prior to curingthe secondary polymer).

Within the secondary component, a first protected or isolated precursorspecies may be present in a certain ratio to a second unprotectedprecursor species so as to provide the correct stoichiometric amounts toallow the reaction to proceed, or some amount more than thestoichiometric amount to ensure full reaction. Such a ratio will dependon the molecular weight of the different species.

According to one or more embodiments in which heat is applied to releasea secondary precursor species (e.g., either by dissolving particles ordegrading capsules), the dissolution or degradation may occur at atemperature selected to be high enough to avoid premature release (e.g.,during fluctuations in shipping and storage temperatures), but not sohigh that the temperature would damage the green structure formed by aphotocured component. In some embodiments the temperature of release maybe between 60° C. and 180° C., between 100° C. and 160° C., or between140° C. and 150° C. Other temperature ranges are also possible.

One issue with using an insoluble particle or capsule component in theliquid resin is the effect it may have on the viscosity of the finalliquid photopolymer. For this reason, the particle or capsule size ofany material considered is an important factor. Particle size affects(a) how well the particles or capsules stay in suspension, and (b)ability to minimize the impact on print layer thickness. In someembodiments, particles or capsules are between 0.1 micron and 200 micronin diameter on average, or between 1 micron and 30 micron in diameter onaverage. Other sizes are also possible. Smaller particles or capsulesmay be more optimal by reducing the viscosity of the resin, remainingbetter suspended, and not interfering with layer thickness.

The compositions and resins described above may be incorporated intomethods for additively manufacturing articles.

Methods may comprise providing a dual-cure resin comprising aphoto-curable component and a secondary component, according to one ormore embodiments of the invention described above.

Methods may further comprise subjecting the photo-curable component toactinic radiation to produce a photo-cured polymer. The step ofsubjecting the photo-curable component to actinic radiation to producethe photo-cured polymer may comprise forming successive layers of thephoto-cured polymer to produce a green article comprising the secondarycomponent.

Methods may further comprise subjecting the secondary component to aninitiating event to allow a previously isolated or substantiallyisolated precursor species to react with a second precursor species toform a secondary cure and provide a manufactured article. The secondarycure may comprise a secondary polymer, such as a polyurea, polyurethane,or epoxy, as described herein. In some embodiments, the initiating eventmay occur while the article is a green article (i.e., after thephoto-curing step).

In embodiments in which a precursor species is contained in a pluralityof particles, the initiating event may be a dissolving event in whichthe particles are dissolved, releasing the precursor species. Inembodiments in which the precursor species is encapsulated, theinitiating event may be a degrading event, in which the capsules aredegraded, releasing the precursor species.

While some of the above description has focused on the application ofheat as an initiating event for causing dissolution of particles ordegradation of encapsulants, it should be understood that othermechanisms may be applied. As discussed above, the initiating event(e.g., dissolving or degrading event) may be accomplished by a varietyof mechanisms: applying an effective amount of heat; applying aneffective mechanical force (e.g., vibrational force); introducing achemical species (e.g., a solvent or a catalyst). In some embodiments,application of an effective mechanical force comprises use of sonicationto degrade an encapsulant (e.g., a polymeric encapsulant) to release asecondary precursor species. Sonication is known to cause degradationand cleavage of polymers. It is possible to selectively adjust theprocess to break particular bonds. It would also be possible tointroduce a chemical species such as a solvent or catalyst to causedegradation of an encapsulant or dissolution of a particle.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated that various alterations,modifications, and improvements will readily occur to those skilled inthe art.

Such alterations, modifications, and improvements are intended to bepart of this disclosure, and are intended to be within the spirit andscope of the invention. Further, though advantages of the presentinvention are indicated, it should be appreciated that not everyembodiment of the technology described herein will include everydescribed advantage. Some embodiments may not implement any featuresdescribed as advantageous herein and in some instances one or more ofthe described features may be implemented to achieve furtherembodiments. Accordingly, the foregoing description and drawings are byway of example only.

Various aspects of the present invention may be used alone, incombination, or in a variety of arrangements not specifically discussedin the embodiments described in the foregoing and is therefore notlimited in its application to the details and arrangement of componentsset forth in the foregoing description or illustrated in the drawings.For example, aspects described in one embodiment may be combined in anymanner with aspects described in other embodiments.

Also, the invention may be embodied as a method, of which an example hasbeen provided. The acts performed as part of the method may be orderedin any suitable way. Accordingly, embodiments may be constructed inwhich acts are performed in an order different than illustrated, whichmay include performing some acts simultaneously, even though shown assequential acts in illustrative embodiments.

Use of ordinal terms such as “first,” “second,” “third,” etc., in theclaims to modify a claim element does not by itself connote anypriority, precedence, or order of one claim element over another or thetemporal order in which acts of a method are performed, but are usedmerely as labels to distinguish one claim element having a certain namefrom another element having a same name (but for use of the ordinalterm) to distinguish the claim elements.

Also, the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having,” “containing,” “involving,” andvariations thereof herein, is meant to encompass the items listedthereafter and equivalents thereof as well as additional items.

What is claimed is:
 1. A dual-cure resin for use in additive manufacturing, comprising: a photo-curable component configured to cure when subjected to an effective amount of actinic radiation; and a secondary component comprising a plurality of encapsulants containing a first secondary precursor species, wherein the plurality of encapsulants are configured to degrade, when subjected to a degrading event, and release the first secondary precursor species for secondary curing.
 2. The dual-cure resin of claim 1, wherein the degrading event comprises application of an effective amount of heat.
 3. The dual-cure resin of claim 1, wherein the degrading event comprises application of an effective mechanical force.
 4. The dual-cure resin of claim 3, wherein the effective mechanical force comprises a vibrational force.
 5. The dual-cure resin of claim 1, wherein the degrading event comprises introduction of a chemical species comprising at least one of a solvent or a catalyst.
 6. The dual-cure resin of claim 1, wherein the encapsulant comprises a wax or polymer shell.
 7. The dual-cure resin of claim 1, wherein the secondary component comprises a second secondary precursor species configured to react with the released secondary precursor species to produce a secondary polymer.
 8. The dual-cure resin of claim 7, wherein the first secondary precursor species comprises at least one chain extender species.
 9. The dual-cure resin of claim 8, wherein the at least one chain extender species comprises at least one of a polyamine species or a polyol species.
 10. The dual-cure resin of claim 7, wherein the second secondary precursor species comprises a diisocyanate species.
 11. The dual-cure resin of claim 1, wherein the secondary component is configured to produce a secondary polymer selected from the group consisting of polyurethane and polyurea.
 12. The dual-cure resin of claim 1, wherein the dual-cure resin is configured to have a shelf-life of at least six months.
 13. The dual-cure resin of claim 1, wherein the dual-cure resin is configured to have a viscosity of between 1 cP and 10,000 cP at 30° C.
 14. A method of producing an additively-manufactured article, comprising: providing a dual-cure resin comprising a photo-curable component and a secondary component, the secondary component comprising a plurality of encapsulants containing a first secondary precursor species, the secondary component further comprising a second secondary precursor species; subjecting the photo-curable component to actinic radiation to produce a photo-cured polymer; subjecting the secondary component to a degrading event to degrade the plurality of encapsulants and release the first heat-curable precursor species; and reacting the first secondary precursor species with the second secondary precursor species to produce a secondary polymer, wherein the photo-cured polymer and the secondary polymer form an additively-manufactured article.
 15. The method of claim 14, wherein subjecting the secondary component to a degrading event comprises applying an effective amount of heat.
 16. The method of claim 14, wherein subjecting the secondary component to a degrading event comprises applying an effective vibrational force.
 17. The method of claim 14, wherein subjecting the secondary component to a degrading event comprises introducing a chemical species comprising at least one of a solvent or a catalyst.
 18. The method of claim 14, wherein the step of subjecting the photo-curable component to actinic radiation to produce the photo-cured polymer comprises forming successive layers of the photo-cured polymer to produce a green article comprising the secondary component.
 19. The method of claim 18, wherein the step of subjecting the secondary component to the degrading event comprises subjecting the green article to the degrading event to degrade the plurality of encapsulants within the green article and release the first secondary precursor species within the green article.
 20. The method of claim 19, wherein the step of reacting the first secondary precursor species with the second secondary precursor species comprises reacting the first secondary precursor species with the second secondary precursor species in the green article to produce the additively-manufactured article. 